Thermionic ionization detectors are used in the field of chromatography for the detection of specific constituent components (i.e., analytes) of a sample that are present in a carrier fluid stream. Such detectors usually include an ionization source having a surface impregnated with an alkali metal compound so as to make the detector specifically sensitive to a halogen, nitrogen, or phosphorus compound. An energy source, such as an electrical heating current carried by a resistive heating wire embedded in the ionization source, heats the ionization source. Certain sample compounds, or their decomposition products, extract the electrical charge from the hot thermionic surface of the source. Ions form at the ionization source and migrate through a fluid stream flowing past the ionization source to a collector electrode. The resulting ion current is collected at the collector electrode. An electronic current-measuring circuit, such as an electrometer, measures the ion current arriving at the collector electrode.
It is known that the sensitivity of a thermionic detector to the presence of analytes in the carrier fluid stream can be degraded by the presence of certain solvents in the carrier fluid stream- For example, the solution of a chlorinated solvent can cause a variation in the baseline output of the detector. The response of the detector to most analytes that are eluted thereafter is inaccurate; the detector is then considered to be unsuitable for most applications. Further, the chemical reaction that occurs between such and the ionization source has been found to damage the ionization source, thus shortening its useful life. Such solvents are accordingly considered herein as "hostile solvents".
Most, if not all, hostile solvents typically exhibit a short retention time and thus their solution can be predicted. One prior art thermionic detection technique therefore attempts to divert the carrier fluid stream from the detector during the elution of the offensive solvents. See, for example, U.S. Pat. No. 3,859,209. Another prior art approach is to decrease the heating current present in the bead when the solvent is eluted. The bead temperature is thereby said to be correspondingly decreased to a temperature that does not support the destructive reaction on the source. See, for example, U.S. Pat. No. 4,202,666.
However, the aforementioned approaches have significant drawbacks. The addition of a valve to divert the carrier fluid stream introduces a dead volume and an additional reactive surface into the fluid stream. This approach is also more costly and complex to implement than is desirable. The practice of lowering the bead temperature has been found to cause the baseline response of the detector to be severely reduced for a delay that is significantly longer than acceptable. That is, the detector output level decreases to an unusable level and the detector output level does not return its original value for as long as a minute or more. Such a delay is beyond the elution time of many of the analytes of interest. As a result, the solution of some analytes cannot be detected with accuracy, and the detector is considered unreliable. Further, the thermal shock that is caused by temperature cycling can shorten the bead lifetime.